I got a great deal on some 4K monitors today, and would like to see if I can run two of them at 4K 60Hz, if that's possible. I am NOT an expert on video cards. I've seen several cards that will supposedly drive a 4K monitor, but there seems to be umpteen variations of each card, each with slightly different model numbers, different amounts of VRAM, etc.

I'm not looking to do any gaming or anything really taxing... probably viewing and maybe editing some 4K video shot on my phone and GoPro. Models that I've seen that supposedly support 4K are:
AMD R9 280x
ATI HD 7950
NVIDIA GTX 960/970/1060/1070

On the NVIDIA, it appears the 10 series has drivers for Sierra and HS. I'm running El Cap right now.

Part of the problems is that I'll see something like AMD R9 280 instead of 280x. Listings will say they only support 2560x1600. I've also looked on barefeats, but I just get confused. Most of the articles are reviewing stuff trying to squeeze the most performance out of a video card. I'm just trying to find a relatively inexpensive card that will run two 4k's at 60Hz. Or at the very least, one 4k at 60Hz, and one at 2550x1600. What should I be looking at? Looks like 3GB VRAM is a minimum. I'd like to stay under $200, so that means buying used, which I'm ok with. Any help is appreciated.